Integrated circuits receive signals through externally accessible input terminals of various designs. In some integrated circuits, the magnitude of the input impedance of input terminals is not critical. Other integrated circuits, particularly memory devices operating at a high speed, the input impedance of at least some of the input terminal must be controlled for optimum performance.
FIG. 1 illustrates a conventional memory device that can advantageously use one or more embodiments of the active termination circuit in according to the present invention. The memory device shown in FIG. 1 is a synchronous dynamic random access memory (“SDRAM”) 10, although the active termination circuit may also be used in other memory devices and other integrated circuits. The SDRAM 10 includes an address register 12 that receives either a row address or a column address on an address bus 14 through an address input buffer 16. The address bus 14 is generally coupled to a memory controller (not shown). Typically, a row address is initially received by the address register 12 and applied to a row address multiplexer 18. The row address multiplexer 18 couples the row address to a number of components associated with either of two memory banks 20, 22 depending upon the state of a bank address bit forming part of the row address. Associated with each of the memory banks 20, 22 is a respective row address latch 26, which stores the row address, and a row decoder 28, which applies various signals to its respective memory bank 20 or 22 as a function of the stored row address. The row address multiplexer 18 also couples row addresses to the row address latches 26 to refresh memory cells in the memory banks 20, 22. The row addresses are generated for refresh purposes by a refresh counter 30 that is controlled by a refresh controller 32.
After the row address has been applied to the address register 12 and stored in one of the row address latches 26, a column address is applied to the address register 12. The address register 12 couples the column address to a column address latch 40. Depending on the operating mode of the SDRAM 10, the column address is either coupled through a burst counter 42 to a column address buffer 44, or to the burst counter 42 which applies a sequence of column addresses to the column address buffer 44 starting at the column address that is output by the address register 12. In either case, the column address buffer 44 supplies a column address to a column decoder 48 which applies various column signals to respective sense amplifiers and associated column circuitry 50, 52 for the respective memory banks 20, 22.
Data to be read from one of the memory banks 20, 22 are coupled to the column circuitry 50, 52 for one of the memory banks 20, 22, respectively. The data are then coupled to a data output register 56 which applies the data to a data bus 58 through a data input buffer 59 and a data output buffer 60. Data to be written to one of the memory banks 20, 22 are coupled from the data bus 58 through a data input register 62 to the column circuitry 50, 52 and then are transferred through word line driver circuits in the column circuitry 50, 52 to one of the memory banks 20, 22, respectively. A mask register 64 may be used to selectively alter the flow of data into and out of the column circuitry 50, 52, such as by selectively masking data to be read from the memory banks 20, 22.
The above-described operation of the SDRAM 10 is controlled by a command decoder 68 responsive to high level command signals received on a control bus 70 and coupled to the command decoder through a command input buffer 72. These high level command signals, which are typically generated by a memory controller (not shown in FIG. 1), are a clock enable signal CKE*, a clock signal CLK, a chip select signal CS*, a write enable signal WE*, a column address strobe signal CAS*, and a row address strobe signal RAS*, with the “*” designating the signal as active low or complement. The command decoder 68 generates a sequence of command signals responsive to the high level command signals to carry out the function (e.g., a read or a write) designated by each of the high level command signals. These command signals, and the manner in which they accomplish their respective functions, are conventional. Therefore, in the interest of brevity, a further explanation of these control signals will be omitted
Each of the input buffers 16, 59, 72 includes a respective termination circuit 90 that is coupled to a respective externally accessible input terminal and that determines the input impedance of the input buffer. Conventional termination circuits 90 include, for example, resistors as well as NMOS and PMOS transistors that are biased to an ON condition. In the past, it has been difficult to efficiently control the input impedance of the input terminals. The resistance provided by transistors and other components can vary with process variations and operating temperature, thus making it difficult to precisely control input impedance. Process variations can be compensated for to some extent by altering the circuit topography during manufacturer using fusible links and the like. However, compensating for processing variations in this manner increases the number of components included in the termination circuit and may increase the number of manufacturing steps. Furthermore, compensating for process variations in does not compensate for temperature variations. Therefore, the input impedance can vary with changes in temperature. Another problem with conventional termination circuits using PMOS or NMOS transistors is that the effective impedance of the transistor varies with the source-to-drain voltage, thus making the impedance of the transistor sensitive to variations in the supply voltage.
A relatively complex circuit (not shown) can be used to implement an active termination circuit 90 that precisely controls the input impedance. However, providing a relatively complex termination circuit 90 for each of the many input terminals of a conventional integrated circuit, such as the SDRAM 10, greatly increases the amount of circuitry in the integrated circuit.
There is therefore a need for a circuit and method that uses relatively little circuitry and yet is able to precisely control the input impedance of an input terminal despite process, temperature and supply voltage variations.